service,manager: New API for mult-client apps

We modified `Manager.GetClient` method in 3e1d9bf4 (service: Allow
multiple clients on the same connection) and while that made things
possible for the Flatpak location portal, it meant us creating
redundant clients for existing system apps (e.g gnome-settings-daemon) and
we don't want that.
This patch changes `GetClient` to the old behaviour of reusing existing
clients and adds a new method `CreateClient` that always creates a new
client object.
Fixes #96.
Also related: #79.